1. Technical Field
This invention relates to radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen.
2. Background Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
A recorded image, such as an X-ray, can be reproduced by stimulating an exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image. There exists in the prior art various scanners for use in reading an image from a stimulable phosphor plate; for example Exelmans (U.S. Pat. No. 5,548,126) describes a scanner for use in a digital radiography system. As discussed in Exelmans, a certain type of phosphor can be energized to an excited state by exposure to X-rays, and then can be stimulated by visible or infrared light (i.e. light of a first sense) to emit visible light in the blue region of the spectrum (i.e. light of a second sense). Other separate senses of light such as polarization state can be used in lieu of wavelength to discriminate between stimulation and emission light.
Typically, light emitted by the phosphor screen upon stimulation is detected by means of an array of charge coupled devices. The light, which is used for stimulating the phosphor screen, has to be separated from the light emitted by the screen upon stimulation.
In order to capture the image stored within the phosphor, one must capture the light of the second sense without contaminating it with backscatter light of the first sense. One possible way to avoid such contamination is to use the decay-time of the phosphor to discriminate, via gating, between the two light senses described above. However, as discussed in Leblens (U.S. Pat. No. 6,228,286), reliance on decay-time, can limit the throughput of digital radiography system.
One issue with a wavelength based system is the need to maximize signal-to-noise ratio (S/N), and therefore requiring the rejection, such as by filtering, of backscatter light in the stimulation wavelength band while maximizing the amount of light captured in the desired emission wavelength band. For example, Struye (U.S. Pat. No. 6,495,850) states that the optical density of the filter at the stimulation wavelength range should be at least 8 while the transmission at the emission wavelength should be higher than 75%. Therefore, there is a tendency, in the prior art, to separate the stimulation and emission wavelength bands to accommodate such characteristics as the filter roll-off.
While this helps in discrimination, it complicates the optical system due to dispersion effects, such effects are discussed in Modern Optical Engineering, W. J. Smith, ISBN 0-07-136360-2. In an optical scanner, such as described in the specification below, such dispersion effects can result in uneven stimulation of an information bearing target area of the image plate and a reduction in quality of the information transferred from the image plate to an optical sensor. Struye further teaches that in order to maximize collection efficiency for an image scanner a large solid angle of the emission must be captured. However, allowing such a large solid angle to pass through an optical filter requires the use of absorptive rather than thin film coated filters, which poses significant materials challenges in meeting the emission wavelength transmission of 75% while rejecting the stimulating wavelengths at very high optical densities.
There exists a need for a scanning system for radiation image recording systems that can high provide a high collection efficiency of the emission wavelengths while rejecting a high degree of the stimulating wavelength.